kern/150920: [ixgbe][igb] Panic when packets are dropped with header split disabled

[Andrew Boyer]

>Number:         150920
>Category:       kern
>Synopsis:       [ixgbe][igb] Panic when packets are dropped with header split disabled
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Fri Sep 24 15:50:01 UTC 2010
>Closed-Date:
>Last-Modified:
>Originator:     Andrew Boyer
>Release:        7.1p3
>Organization:
Avere Systems Inc.
>Environment:
amd64
>Description:
The ixgbe and igb drivers have *_rx_discard methods which attempt to reuse mbufs in the rx_ring.  They fail to check whether the mbuf pointers are valid before dereferencing them, though.  This causes a panic when header split is disabled, because the rbuf->m_head pointer will always be NULL.
>How-To-Repeat:
Found by adding a failure point in ixgbe_rxeof() right before the staterr check.  The first packet that's rejected causes a panic.
>Fix:
Check for NULL pointers before dereferencing mh and mp in ixgbe_rx_discard() and igb_rx_discard().

I don't see any other places where m_head is used without being checked.

>Release-Note:
>Audit-Trail:
>Unformatted: